This invention relates to an unsaturated polyester resin composition which possesses excellent low shrinkage and also excellent mechanical strength.
Sheet molding compounds (hereinafter referred to as SMCs), bulk molding compounds and the like, which are prepared by blending unsaturated polyester resins with fillers, pigments, thickeners, fiber reinforcing materials, hardening agents, stabilizers, mold releasing agents and the like, have recently been used in increasing amounts as new industrial materials. These compositions fulfill the requests for materials capable of reducing the labor cost and improving the productivity and the working environment in the reinforced plastic industry.
However, molded products of unsaturated polyester resins tend to have cracks, distortions, warps, waviness and the like, since the resins show large shrinkage on hardening and the shrinkage causes embossings of reinforcing agents and peelings at the interface. In addition, it is difficult to produce molded products of high dimensional precision because of the shrinkage on hardening.
For improving these drawbacks, the practice has been generally adopted of blending thermoplastic resins or saturated polyesters with the unsaturated polyester resins. There have been proposed various unsaturated polyester resin compositions, for example, an unsaturated polyester resin composition comprising (1) a reaction product of an unsaturated monoglycidyl compound with a saturated polyester which consists of a saturated polycarboxylic acid and polyhydric alcohol, and which has a molecular weight of 3,000-50,000, (2) an unsaturated polyester resin and (3) a monomer copolymerisable with the said (2) unsaturated polyester resin (Japanese Patent Laid-Open Publication No. 52-3 9785). Another such composition comprises an unsaturated polyester resin composition which is produced by blending an unsaturated polyester alkyd with a liquid unsaturated polyester resin which is produced by first esterifying a dicarboxylic acid mixture of not less than 50 mole % of an unsaturated dicarboxylic acid and not more than 50 mole % of a saturated dicarboxylic acid with a glycol, and second by blending the esterified product with a vinyl monomer crosslinking agent. Said unsaturated polyester alkyd is prepared by esterifying a dicarboxylic acid mixture of not more than 10 mole % of an saturated dicarboxylic acid and not less than 90 mole % of a saturated dicarboxylic acid, with a glycol (Japanese Patent Laid-Open Publication No. 50-5 1188). A third such composition is an unsaturated polyester resin composition comprising at least two unsaturated resins different in polymerization characteristics (Japanese Patent Laid-Open Publication No. 116189/1974).
In these processes, there are drawbacks, such as the blended thermoplastic polymers or the saturated polyesters do not participate in the crosslinking upon curing, with the result being that they exist in the molded product in a separated state and accordingly the molded product is weakened in the mechanical strength thereof and the additives therein are forced up to the surface of the molded product, making its appearance remarkably poor.